How does i convert vector to scaler value

Course Q&A Machine Learning Specialization Advanced Learning Algorithms
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in this image, we compute g(w* a +b ) and the output is scaler value not vector
so how does layer compute g(w* a +b ) → 1/1+e power(-z)
and convert (a) vector to scaler value (output of neural network)

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In general, w and a are vectors, so the multiplication operator is a dot product. The result of the dot product of two vectors is a scalar (by definition).

This result is then passed through the function g(), which traditionally means sigmoid.